1. Field of the Invention
This invention relates to a process for the preparation of bismuth oxide, a method for the formation of a bismuth layered oxide, and a method for the fabrication of a capacitor structure of a semiconductor device.
2. Description of the Related Art
In recent days, application researches of a non volatile memory cell which adopts a ferroelectric thin film have been briskly conducted accompanied with an advance in the film forming technique. This non volatile memory cell is a non volatile memory cell which makes use of a high-speed polarization inversion of a ferroelectric thin film and its remanence and is high-speed reloadable. The non volatile memory cell adopting a ferroelectric thin film, which cell is under study now, can be classified into two systems, that is, a system of detecting a change in the accumulated charge amount of a ferroelectric capacitor and a system of detecting a change in resistance caused by the spontaneous polarization of the ferroelectric substance. The semiconductor device according to the present invention belongs to the former one.
At high temperatures, triphenyl bismuth vaporizes accompanied with decomposition so that the temperature control for maintaining a stable vapor amount is very difficult. For example, according to Literature 1, that is, "Ferroelectric bismuth titanate films by hot wall metalorganic chemical vapor deposition" in J. Si, et al., "J. Appl. Phys. 73(11), 7910-7913(Jun. 1, 1993)", it is necessary to maintain a container for raw materials at a temperature of from 165 to 170 in order to secure a sufficient vapor pressure of triphenyl bismuth. When the container for raw materials is maintained at such a high temperature, the following disadvantages occur.
(A) Triphenyl bismuth in the container for raw materials is gradually decomposed when exposed to high temperatures for a long time, so that it becomes difficult to continue a stable gas supply.
(B) It is necessary to heat each part of the MOCVD apparatus such as a container for raw materials and piping to 150-200.degree. C. in order to conduct efficient gas supply and besides, to prevent recoagulation in the piping during transportation to the MOCVD reaction chamber. It is however difficult to conduct temperature control or temperature management for the above purpose.
Triphenyl bismuth is a relatively stable substance. Its reactivity is, on the other hand, low so that the concentration of its component cannot easily be increased at the time of forming a ferroelectric thin film by the MOCVD method. In addition, triphenyl bismuth and its analogous compounds are not advantageous for the formation of its oxide film because they do not have a metal-oxygen bond in their molecules. When a ferroelectric thin film, which is an oxide film, is formed on the surface of a base material, both of the decomposition reaction of triphenyl bismuth and the reaction with oxygen are necessary as the reaction steps on the surface of the base material. Accordingly, the use of the above-described source raw materials requires the external addition of an oxidation source. The preparation of bismuth oxide using triphenyl bismuth is conducted as follows: EQU --Bi--C.sub.6 H.sub.5 .fwdarw.--Bi--+C.sub.6 H.sub.5 -- EQU --Bi--+O.fwdarw.--Bi--O--
Described specifically, when bismuth oxide is prepared using triphenyl bismuth, the reaction is conducted through two steps. So, the preparation of bismuth oxide or formation of an oxide film containing bismuth as a component element is susceptible to the temperature of the base material or the reaction pressure in the MOCVD reaction chamber.